1. Field
The invention relates to a process for the production of nanocrystalline magnet cores as well as devices for carrying out such a process.
2. Description of Related Art
Nanocrystalline iron-based soft magnetic alloys have been known for a long time and have been described, for example, in EP 0 271 657 B1. The iron-based soft magnetic alloys described there have in general a composition with the formula:(Fe1-a Ma)100-x-y-z-αCuxSiyBzM′αwhere M is cobalt and/or nickel, M′ is at least one of the elements niobium, tungsten, tantalum, zirconium, hafnium, titanium, and molybdenum, the indices a, x, y, z, and α each satisfy the condition 0≦a≦0.5; 0.1≦x≦3.0, 0≦y≦30.0, 0≦z≦25.0, 5≦y+z≦30.0, and 0.1≦α30.
Furthermore, the iron-based soft magnetic alloys can also have a composition with the general formula(Fe1-a Ma)100-x-y-z-α-β-γCuxSiyBzM′αM″62 Xγwhere M is cobalt and/or nickel, M′ is at least one of the elements niobium, tungsten, tantalum, zirconium, hafnium, titanium, and molybdenum, M″ is at least one of the elements vanadium, chromium, manganese, aluminum, an element of the platinum group, scandium, yttrium, a rare earth, gold, zinc, tin, and/or rhenium, and X is at least one of the elements carbon, germanium, phosphorus, gallium, antimony, indium, beryllium, and arsenic and where a, x, y, z, α, β, and γ each satisfy the condition 0≦a≦0.5, 0.1≦x≦3.0, 0≦y≦30.0, 0≦z≦25.0, 5≦y+z≦30.0, 0.1≦α≦30.0, β≦10.0, and γ≦10.0.
In both alloy systems at least 50% of the alloy structure is occupied by fine-crystalline particles with an average particle size of 100 nm or less. These soft magnetic nanocrystalline alloys are to an increasing extent used as magnet cores in inductors for the most various applications in electrical engineering. For example, summation current transformers for alternating current-sensitive and also pulse current-sensitive ground fault circuit breakers, chokes and transformers for switched power supplies, current-compensated chokes, filter chokes, or transductors made of strip-wound cores which have been produced from strips made of the nanocrystalline strips described above are known. This follows, for example, from EP 0 299 498 B1. Furthermore, the use of such annular strip-wound cores also for filter sets in telecommunications is known, for example, as interface transceivers in ISDN or also DSL applications.
The nanocrystalline alloys coming into consideration can, for example, be produced economically by means of the so-called quick-hardening technology (for example, by means of melt-spinning or planar-flow casting). Therein an alloy melt is first prepared in which an initially amorphous alloy is subsequently produced by quick quenching from the melted state. The rates of cooling required for the alloy systems coming into consideration above are around 106 K/sec. This is achieved with the aid of the melt spin process in which the melt is injected through a narrow nozzle onto a rapidly rotating cooling roller and in so doing hardened into a thin strip. This process makes possible the continuous production of thin strips and foils in a single operational step directly from the melt at a rate of 10 to 50 m/sec, where strip thicknesses of 20 to 50 μm and strip widths up to ca. several cm. are possible.
The initially amorphous strip produced by means of this quick-hardening technology is then wound to form a geometrically highly variable magnet core, which can be oval, rectangular, or round. The central step in achieving good soft magnetic properties is the “nanocrystallization” of the up to this point amorphous alloy strips. These alloy strips still have, from the soft magnetic point of view, poor properties since they have a relatively high magnetostriction |λS| of ca. 25×10−6. In carrying out a heat treatment for crystallization adapted to the alloy an ultra-fine structure then arises, that is, an alloy structure arises in which at least 50% of the alloy structure is occupied by cubically spatially centered FeSi crystallites. These crystallites are imbedded in an amorphous residual phase of metals and metalloids. The reasons, from the point of view of solid state physics, for the arising of the fine-crystalline structure and the drastic improvement of the soft magnetic properties thus appearing is described, for example, in G. Herzer, IEEE Transactions on Magnetics, 25 (1989), Pages 3327 ff. Thereafter good soft magnetic properties such as a high permeability or low hysteresis losses through averaging out of the crystal anisotropy Ku of the randomly oriented nanocrystalline “structure” arise.
According to the state of the art known from EP 0 271 657 B1or 0 299 498 B1 the amorphous strips are first wound on special winding machines as free from tension as possible to form annular strip-wound cores. For this, the amorphous strip is first wound to form a round annular strip-wound core and, if required, brought into a non-round form by means of suitable forming tools. Through the use of suitable winding elements however, forms can also be achieved directly with winding of the amorphous strips to form annular strip-wound cores which are different from the round form.
Thereafter the annular strip cores, wound free of tension, are, according to the state of the art, subjected to a heat treatment for crystallization which serves to achieve the nanocrystalline structure. Therein the annular strip-wound cores are stacked one over the other and run into such an oven. It has been shown that a decisive disadvantage of this process lies in the fact that by weak magnetic stray fields, such as, for example, the magnetic field of the earth, a positional dependence of the magnetic values is induced in the magnet core stack. While at the edges of the stack for example, there are high permeability values with an intrinsically limited high remanence ratio of more than 60%, the magnetic values in the area of the middle of the stack are characterized by, more or less pronounced, flat hysteresis loops with low values with regard to permeability and remanence.
This is, for example, represented in FIG. 1. FIG. 1a shows the distribution of the permeability at a frequency of 50 Herz as a function of the serial number of the cores within an annealing stack. FIG. 1b shows the remanence ratio Br/Bm as a function of the serial number of the cores within an annealing stack. As can be seen from FIGS. 1a and 1b, the distribution curve for the magnetic values of an annealing production lot is broad and continuous. The distribution curve drops off monotonically at high values. The precise specific curve depends there on the alloy, the magnet core geometry, and naturally the height of the stack.
In the case of the nanocrystalline alloy structures in question the onset of the nanocrystalline structure typically occurs at temperatures of Ta—450° C. to 620° C., where the necessary hold times can lie between a few minutes and ca. 12 hours. In particular, it follows from U.S. Pat. No. 5,911,840 that in the case of nanocrystalline magnet cores with a round BH loop a maximal permeability of μmax=760,000 is reached when a stationary temperature plateau, with a duration of 0.1 to 10 hours below the temperature required for the crystallization of 250° C. to 480° C., is used for the relaxation of the magnet cores. This increases the duration of the heat treatment and reduces its economy.
The present invention is based on the discovery that the magnetostatically related formations of parabolas shown in FIGS. 1a and 1b in the stack annealing of annular strip-wound cores in retort ovens are of a magnetostatic nature and are to be traced back to the location-dependence of the demagnetization factor of a cylinder. Furthermore, it was determined that the exothermic heat of the crystallization process increasing with the core weight can only be released to the environment of the annealing stack incompletely and thus can lead to a clear worsening of the permeability values. It is noted that the nanocrystallization itself is obviously an exothermic physical process. This phenomenon has already been described in JP 03 146 615 A2. The consequence of this insufficient drain of the heat of crystallization is a local overheating of the annular strip-wound cores within the stack which can lead to low permeabilities and to higher remanences. Accordingly, the permeabilities and the remanences of cores in the center of the annealing stack are lower than the permeabilities and the remanences of annular strip-wound cores at the outer edge of the annealing stack. Previously one got around this problem, to the extent that one recognized it at all, by, e.g. as in U.S. Pat. No. 5,911,840, by applying heat, in an uneconomical manner, very slowly in the range of the onset of nanocrystallization, that is, ca. 450° C. Typical heating rates lay in this case between 0.1 and 0.2 K/min, due to which running through the range up to the temperature of 490° C. alone could take up to 7 hours. This method of processing was very uneconomical.
It is thus the objective of the present invention to provide a new process for the production of annular strip-wound cores in which the problem stated initially of dispersion in the form of a parabola and other, in particular exothermically related, worsenings of magnetic indices can be avoided and which works particularly economically.